Close Coupled—centrifugal pumps do not, as already mentioned above, have separate bearings for the shaft of the impeller, but the bearings of the electric motor carry all the loads subjected to the shaft, i.e. both radial and axial forces as well as the weight of the impeller and the shaft end hanging outside the housing or bearings of the electric motor. In close coupled centrifugal pumps the shaft of the electric motor may extend to such a length that the centrifugal impeller is fastened directly to the end of the shaft of the electric drive motor. Another exemplary alternative is that the electric drive motor is a standard one, based for instance on IEC- or NEMA-standards, having a substantially short shaft extending out of the motor housing. In the latter case the shaft of the impeller or, in fact, of the centrifugal pump is arranged as an extension of the shaft of the electric motor. In other words, the shaft of the centrifugal pump is by some appropriate means, like flange coupling or shrink fitting, coupled or fastened to the end of the shaft of the electric motor.
A problem relating to the use of drive motors in accordance with IEC- or NEMA-standards is the adjustment of the running clearance of the impeller of the centrifugal pump, especially in relation to the rear wall or casing cover of the centrifugal pump. A normal option would be to provide the various factors, i.e. the length of the shaft of the centrifugal pump and the length or dimension the shaft of the electric motor extends out of the housing of the electric motor, having an effect on the running clearance with proper tolerances, but the tolerances, or the tolerance range, for the length or dimension of the shaft of the electric motors manufactured in accordance with the standards, are too wide. In other words, as long as standard electric motors are used for driving centrifugal pumps, the shaft construction of the electric motors cannot be changed. In some cases this results in the running clearance of the impeller being too wide in spite of the fact that the shaft of the centrifugal pump is manufactured as accurately as it is normally done.
The running clearance has to be wide enough so that the impeller does not get into physical contact with the rear wall or the casing cover of the pump. This kind of a structure leads in the reduction in both the efficiency and the head of the pump.
GB 972,321 discusses a centrifugal pump drive motor combination where the shaft of the centrifugal pump impeller is supported by the shaft of the electric drive motor. The supporting is realized by providing the drive motor shaft with a hollow internally splined end portion, and the centrifugal pump shaft by means of an externally splined end portion that fits into and cooperates with the hollow end portion of the drive shaft. On the one hand, the drive motor shaft is provided with a long screw extending through the shaft such that by means of the screw the centrifugal pump shaft may be drawn towards the drive motor shaft. And on the other hand, the centrifugal pump shaft is provided with threads beside the splined end portion such that a nut arranged on the threads cooperates with the end surface of the drive shaft and tends to move the centrifugal pump shaft away from the drive shaft. The axial adjustment of the centrifugal pump shaft is performed by means of, for instance, adjusting the correct impeller clearance by threading the nut, whereafter the screw is tightened, thus locking the axial position of the centrifugal pump shaft.
U.S. Pat. No. 4,118,142 discusses an adjustable coupling for operatively connecting the shaft of a low-pressure deep well pump to the shaft of a high-pressure booster pump whereby both pumps may be driven by a common prime mover. The adjustable coupling is formed of a coupling assembly fastened to the end of a hollow electric motor drive shaft arranged on and at a clearance to the shaft of the pump impeller. The clutch assembly is coupled to the impeller shaft by means of a key, which allows the impeller shaft to be moved axially in relation to the drive shaft. The impeller shaft is provided with threads, which cooperate with a nut so that when threading the nut the impeller shaft is moved in relation to the drive shaft. After a correct position of the drive shaft is found the nut is locked by means of screws to the clutch assembly.